Question

Analysis Questions

1. Polarizing disks, such as the ones used in this experiment, were invented by Dr. Edwin Land. You will recall that the long-chain hydrocarbon molecules in the disk material are made to be electrically conducting. In what direction is the transmission axis for such a disk, relative to the long molecules?

Synthesis Questions

1. In everyday life, radio waves are typically polarized, but light waves are not. Why is this the case?

2. The glare of headlights of oncoming traffic causes many automobile accidents every year. Would it be possible to design a “non-glare” headlight system for cars?

Answer 1

Analysis Question

1.

Dr. Edwin Land, in his experiments, constructed linear polarizers. These polarizers are sheets of a certain material that allow the light waves oscillating in a specific direction to pass through them. All waves oriented in a different direction are selectively absorbed. He created these sheets using long chain hydrocarbons. The sheets were stretched during manufacture such that the hydrocarbon chains got aligned in a specific direction. Post stabilization, the sheets were treated in a halogen solution (e.g.-Iodine). This treatment imparted electrical conductivity to the chains, since halogen ions have one excess electron. The conductivity due to these electrons is unlike conductivity in metals. In metals, the conductivity is due to free electrons which are de-localized and detached from the metal atoms. These free electrons can move about anywhere within the confines of the solid. The conductivity in the polarizer sheets is due to valence electrons only along the hydrocarbon chains. In other words, the electrons due to the halogens cannot move about freely, but can propagate along the chains of the material.

Now, if we were to incident unpolarized light (electric and magnetic fields oscillating in multiple planes) onto a sheet such as this, the conduction electrons in the chains would be able to absorb electric fields oscillating along the chains or parallel to the chains. Therefore, the electric fields that will be allowed to pass through the material are the once that oscillate perpendicular to the orientation of the chains. In conclusion. the direction of the transmission axis in such a disk or sheet is perpendicular to the orientation of the long chain molecules in it.

Synthesis Questions

1.

Radio waves lie at the very end of the EM spectrum with a very high wavelength range: 1 mm to 100 km. Whereas, the wavelength of visible light ranges from 400 nm (blue) to 800 nm (red). EM waves can get scattered by particles that have a size comparable to the wavelength of the respective wave. The scattered waves get deviated from their path and the plane of vibration also changes. This is why visible light is scattered easily. The particles in our atmosphere scatter blue light to the largest extent (thus the sky is blue). However red light, or light in the red region of the spectrum get scattered to a minimum. Thus, one can infer that waves with higher wavelength get scattered less (there are no particles large enough in the atmosphere to scatter radio waves).

Radio waves are widely used in signal transmission since they do not get scattered easily. When radio waves emitted by antennae of a specific polarization, these waves can only be received by antennae with the same polarization. Alternatively, if one were trying to receive radio signals with an antennae oriented perpendicular to the polarization of the signal, they would read nothing. Such polarized and coherent radio waves are used in radar, navigation systems, broadcasting, radio astronomy (radio telescopes).

Visible light from most day to day sources are de-coherent and unpolarized. This is not a problem however. We can see visible light. Our real life detectors of visible light are our eyes, cameras, etc. These detectors do not have a very long range since visible light does not have a very long range. Our detectors, even though sensitive to polarization, do not need a specific polarization for functioning. For our eyes, polarization of light would create a lowering in intensity, but we don't need critically accurate signals for functioning.

2.

Glare is the blinding or dazzling effect of a very intense source of light incident directly or indirectly on our eyes. Creating anti-glare headlights is a possible solution to this problem. One of the ways this can be done is explained briefly below:

A camera in front of the car captures the image of the oncoming traffic. A sensor detects the bright spots (headlights) in the image from the camera. Using a pair of bright spots of light, the sensor can approximately triangulate the position of the head of the driver of the oncoming vehicle. Now, instead of using regular headlights, one can use a projector, that illuminates everything in front of the vehicle except for the drivers head. This would prevent the other driver from experiencing glare.

This is just an example of how the problem might be mitigated, not the ultimate solution. Also, note that this method would protect the other driver from glare and not oneself. A much more feasible method of eliminating glare is by the use of anti-glare glasses while driving. These glasses are simply polarized glasses. As stated before, these glasses help cut down the intensity of incident light. Polarized glasses are usually circular polarizers that allow either clockwise or anti-clockwise polarized light to pass, as opposed to linear polarizers that allow a specific plane of light to pass.